As an alternative to Public Switched Telephone Network (PSTN) systems, cellular phone networks have proliferated over the last decade where users with cellular phones have access to one or more networks at almost any location. Also a recent development is the wide spread use of Voice over IP (VOIP) telephony, which uses internet protocol over wired and wireless networks. With the availability of such diverse types of communication networks and devices capable of taking advantage of various features of these networks, a new technology is bringing different communication networks together providing until now unavailable functionality such as combining various modes of communication (e.g. instant messaging, voice calls, video communications, etc.). This technology is referred to as unified communications (UC). A network of servers manages end devices capable of handling a wide range of functionality and communication while facilitating communications between the more modern unified communication network devices and other networks (e.g. PSTN, cellular, etc.).
Complex functionalities such as multi-modal communication, presence based features, dynamic attribute assignment, and preservation of conversations are accommodated by UC systems taking advantage of server/network functionality and end point device capabilities. Since communication between components of such networks involves much more than simple exchange of digital or analog voice signals, enhanced data exchange protocols (at various layers) are employed like Session Initiation Protocol, Session Description Protocol, and comparable ones.